It has long been known that the efficiency of a catalyzer deteriorates with increasing operating age. Various methods and arrangements are known to determine the state of deterioration of the catalyzer. Most of these methods and arrangements establish a relationship between a measuring signal and a test signal to determine the value of a deterioration variable with the measuring signal being detected by a lambda probe mounted forward of the catalyzer and the test signal being detected by a lambda probe mounted rearward of the catalyzer. For example, U.S. Pat. No. 3,962,866 discloses a catalyzer monitoring system wherein the deterioration state is defined with the aid of the difference between the amplitudes of these two signals. A warning signal is emitted as soon as the difference drops below a threshold value. German published patent application 3,500,594 discloses an arrangement wherein the ratio of the amplitudes is used in lieu of the difference with the signal being, in addition, averaged.
Forming a relationship between the measuring signal and the test signal permits the influence of various operating conditions on the deterioration variable to be reduced. If a measuring signal having a large amplitude occurs forward of the catalyzer, that is exhaust gas is supplied to the catalyzer having a large deviation from the lambda value one, it is understood that the amplitude of the lambda signal at the output of the catalyzer also increases since, with the same storage capacity of the catalyzer, that much more unconverted toxic gas exits from the catalyzer the greater the amount supplied at the input of the catalyzer. By means of establishing the above-mentioned relationship and especially the quotient formation, the influence of a large measuring signal on the deterioration variable is greatly reduced.
However, it has been shown that the above-mentioned measures notwithstanding, there are still large fluctuations of the computed deterioration variable which have been observed. The reason for this will be explained in greater detail below. In any event, it has long been a desire to reduce these fluctuations in the deterioration variable by an appropriately configured method and an appropriately configured arrangement.